The natural sense of hearing in human beings involves the use of hair cells in the cochlea that convert or transduce acoustic signals into auditory nerve impulses. Hearing loss, which may be due to many different causes, is generally of two types: conductive and sensorineural. Some types of conductive hearing loss occur when the normal mechanical pathways for sound to reach the hair cells in the cochlea are impeded. These sound pathways may be impeded, for example, by damage to the auditory ossicles. Conductive hearing loss may often be overcome through the use of conventional hearing aids that amplify sound so that acoustic signals can reach the hair cells within the cochlea. Some types of conductive hearing loss may also be treated by surgical procedures.
Sensorineural hearing loss, on the other hand, is caused by the absence or destruction of the hair cells in the cochlea, which are needed to transduce acoustic signals into auditory nerve impulses. People who suffer from severe to profound sensorineural hearing loss may be unable to derive significant benefit from conventional hearing aid systems, no matter how loud the acoustic stimulus. This is because the mechanism for transducing sound energy into auditory nerve impulses has been damaged. Thus, in the absence of properly functioning hair cells, auditory nerve impulses cannot be generated directly from sounds.
To overcome sensorineural hearing loss, numerous cochlear implant systems—or cochlear prostheses—have been developed. Cochlear implant systems bypass the hair cells in the cochlea by presenting electrical stimulation directly to the auditory nerve fibers by way of an array of electrodes implanted within the cochlea. Direct stimulation of the auditory nerve fibers leads to the perception of sound in the brain and at least partial restoration of hearing function.
In a typical cochlear implant system, a cochlear implant operates in accordance with radio frequency (“RF”) power inductively provided by an externally located sound processor. It is often desirable to minimize the amount of RF power that is provided to the cochlear implant so that battery life of the sound processor may be maximized. To this end, the sound processor may perform an RF power level and internal loading sequence during which a table (e.g., a power estimator (“POEM”) table) is built that defines RF power levels required to apply different current levels to a particular electrode. During this RF power level and internal loading sequence, the cochlear implant may use a multiplexer to create a load that is parallel to an electrode and that simulates an impedance associated with the electrode. Current generated by a current source associated with the electrode may then be applied to the load. While the current is being applied, the sound processor may determine an RF power level required to generate and apply the current. Unfortunately, because the electrode is in parallel with the load created by the multiplexer, some of the current is also applied to the electrode. This may result in the patient perceiving a pulsing noise, which can be annoying and/or disconcerting to the patient.